1. Field of the Invention
This invention relates generally, to current transformers utilized as voltage and current sensors, and more particularly to voltage and current sensors for making accurate measurements from an rf conductor in the presence of external electric and magnetic fields.
2. Brief Description of the Prior Art
It is well know by those skilled in the art that the typical toroidal transformer used to measure current in a conductor has a combined inductance and capacitance associated with the current transformer windings that can make the transformer ineffective at high frequencies. This is especially true when the transformers are used as voltage and current sensors in high frequency plasma processing applications. It is also well known that the orientation of a Faraday shielded torroidal current transformer is relatively impervious to the presence of external electrical and magnetic fields.
One attempt to overcome these problems was disclosed in the U.S. Pat. No. 3,146,417 issued to Pearson on Aug. 25, 1964. This patent teaches a transformer which includes a core having a plurality of windings wound around the core. A load resistor R is formed as a distributed resistor that has a plurality of taps connected to portions of the windings of the transformer. Each of the sections of winding and transformer included between two taps is now itself a small transformer whose contribution to the total output voltage is proportional to the fraction of the transformer determined by the number of turns and the proportion of resistance and capacitance between the taps. The Pearson patent teaches that the effect that the current transformer load resistance has upon the high frequency response and transient performance of the transformer depends upon the ratio of resistance to inductance of the load resistance. The higher the ratio, the better the high frequency and transient response.
Although, the Pearson patent demonstrated a device that had higher frequency response, it had several major drawbacks. The windings of the transformer had to be hand wound that was both labor intensive and costly. Also, the quality in the manufacture of the device could not be uniformly maintained from sensor to sensor.
U.S. Pat. No. 5,770,992 teaches a current sensor composed of traces embedded in a circuit board on one side of an RF conductor and a voltage sensor composed of a separate circuit board on the other side of an RF conductor. It is well know by those skilled in the art that a non-torroidal current transformer used to measure current in a conductor can make the transformer susceptible to external magnetic fields.
It would be desirable if there were provided a voltage and current sensor that could reliably make measurements that was easy to manufacture and was not susceptible to external magnetic and electric fields.
There is provided by this invention a voltage and current sensor that is not susceptible to external magnetic and electric fields. The sensor is generally comprised of a two-layer printed circuit board with multiple surface mounted inductors and resistors in parallel around the circumference of a conductor to sense the current and voltage of the conductor. A plated aperture used as a voltage sensor is recessed in the board to be completely shielded from the external influence of voltage fields.